Flux cored electrodes have been used for many years and include an outer metal sheath sized around an inner core of particles forming a flux system. To control the composition of the weld metal formed by using the electrode in an electric arc welding process, the core generally includes a number of metal particles to be melted and alloyed into the weld metal resulting from the welding process. It is common practice to use magnesium particles in the core of the electrode so these particles are evenly dispersed with the other core particles to produce a flux cored electrode. The use of small magnesium particles in flux cored electrodes is common.
The flux in a cored electrode that produces high impact welds while welding vertically up or overhead and has high melt off rates is disclosed in Amata U.S. Pat. No. 4,551,610. Particles of lithium oxide, iron oxide, silicon dioxide, lithium carbonates, magnesium and aluminum are used in the core. This patent is incorporated by reference herein to illustrate a representative flux cored electrode using magnesium. The magnesium particles can be particles employing the present invention. This patent also combines a large amount of elemental aluminum with a smaller amount of elemental magnesium to provide the oxidizing agents in the weld metal for a self shielded electrode. The present invention is primarily used for gas shielded electrodes, but also relates to the concept of using a combination of magnesium with standard aluminum particles. Thus, this patent teaches a type of electrode in which the present invention can be implemented.
In Sakai U.S. Pat. No. 4,571,480, another flux cored electrode using aluminum and magnesium particles is disclosed. This flux cored electrode patent discusses the function of magnesium particles and the combination of aluminum and magnesium in the core of the electrode. However, there is an indication that the magnesium leads to an increase in fume generation. Thus, it is suggested that magnesium particles should be in an amount not more than 10%. The magnesium vaporizes into an explosive substance upon exposure to heat of the arc. The explosive nature of the magnesium causes formation of a large number of spatter events. Thus, it is preferable that magnesium particles are in the form of a magnesium alloy, such as aluminum-magnesium, magnesium-silicon, magnesium-silicon calcium, nickel-magnesium or lithium-magnesium. Disadvantages of using magnesium particles, as discussed in the Sakai patent, are overcome by the formation of magnesium powder in accordance with the present invention. However, this patent does illustrate the problems to which the present invention is directed and the function of magnesium and the function of a magnesium oxide which prevents magnesium from being a deoxidizing or denitrification material in the molten weld metal. Thus, this patent teaches the reasons for elemental magnesium and slag forming magnesium oxide, together with some of the difficulties associated with using the elemental magnesium particles in the flux cored electrode. Indeed, the solution suggested in this prior art is the use of aluminum as an alloy with elemental magnesium metal. By using the present invention, modification of magnesium into an alloy particle before inclusion into the core of the electrode is not required. Another background flux cored electrode is described in Sakai U.S. Pat. No. 5,580,475 which again discusses the advantage of magnesium particles wherein cracking of the metal is reduced, together with the improved slag removability using magnesium oxide or magnesium particles in an amount of about 0.2%. This use of deoxidizing magnesium or slag forming magnesium oxide causes deterioration of the bead shape, but adds to the improved slag removability. Thus, there is a need to reduce both the amount of magnesium in the flux core and the amount of resultant magnesium oxide formed from magnesium during or before the welding process. Magnesium acts to reduce oxygen in the weld metal. MgO also removes free oxygen. These two magnesium sources are effective to improve toughness and blow hole resistance. The preferred source of magnesium is metal or elemental magnesium particles even though, as described, metal alloys of magnesium can be employed. In this patent, the magnesium oxide is incorporated into the flux core as a separate oxide so it performs its function in the welding process as the magnesium particles react actively as a deoxidizing agent. The magnesium and magnesium oxide improve the slag removability. Thus, there is a need to provide both magnesium in the metal form without requiring an alloy and magnesium as magnesium oxide. These two Sakai patents are incorporated by reference herein as illustrating electrodes which can employ advantageously magnesium particles of the present invention.
As described in the various prior art patents, magnesium particles are used in the core of flux cored electrodes to deoxidize the weld metal during the welding process. However, magnesium forms hydrides due to the inherent reactivity of magnesium. Consequently, as the cored electrode is manufactured by being extruded to size and then baked to a temperature of 400-700° F., the reactive magnesium particles tend to be hydrated. Furthermore, hydration occurs as the reactive particles are stored awaiting filling into the metal sheath prior to the extrusion and baking. Since the procedure for forming flux cored electrodes involves exposure to the atmosphere and high heat, the magnesium small particles in the core are also oxidized into MgO. This conversion changes the reactivity of the magnesium and decreases its oxidizing capability in the molten weld metal.
The baking process is required to remove drawing lubricants from the extruded electrode. By hydration of the magnesium particles and oxidation of the magnesium particles, the magnesium particles in the core material has a reduced amount of active magnesium available for its primary function of deoxidation in the weld metal. In other words, the amount of available magnesium for deoxidation prior to the electrode manufacturing process is substantially greater than the actual magnesium available in the final flux cored electrode. Some of the magnesium is converted to MgO or is hydrated by hydrophilic action of the very reactive, small magnesium particles. Hydration of the magnesium powder during storage and baking, reduces the amount of magnesium available for oxidation by producing a certain amount of MgH2. This hydrogen compound decomposes in the molten metal to increase the amount of diffusible hydrogen in the weld. Thus, the formation of MgH2 during processing or storage increases the tendency of the magnesium powder itself to cause higher levels of diffusible hydrogen. Furthermore, the oxidation of the magnesium particles into MgO reduces the amount of deoxidation potential of the particles in the core. Thus, the use of small magnesium particles in the past has involved balancing the advantage of magnesium to deoxidize the molten metal with the disadvantage of hydrogen pick-up of the magnesium powder. The hydrogen pick-up, together with oxidation of the magnesium powder during processing, reduces the deoxidation potential of the magnesium particles. The present invention reduces these disadvantages of using small magnesium powder in the flux core of an electrode. When the electrode is gas shielded, the magnesium particles are normally less than about 25% and there is no added aluminum powder. On the other hand, when the flux cored electrode is used for self shielding, the core includes small aluminum particles together with small magnesium particles, with the aluminum particles being greater in weight. The aluminum and magnesium particles form the deoxidation component of the core for self shielded electrodes. The present invention is primarily directed to gas shielded electrodes wherein the core merely includes small magnesium particles for deoxidation. However, the invention is also capable of being employed in self shielded flux cored electrodes where small aluminum and small magnesium powders are used for deoxidation. The term small means less than about 0.025 inches in major dimension. This particle size allows better distribution in the core.